The present invention relates to an internal combustion engine equipped with an exhaust gas recirculation device that recirculates part of exhaust gas from an exhaust passage downstream of an exhaust gas purifier into an intake passage as EGR gas.
Exhaust gas from an internal combustion engine contains particulate matter or PM mainly composed of soot and unburned fuel. There are known exhaust gas purifiers such as a diesel particulate filter or DPF provided in an exhaust passage of the engine to collect PM in the exhaust gas and as a DPNR system provided in the exhaust passage and having such filter and a NOx storage and reduction catalyst. These exhaust gas purifiers need to be regenerated at an appropriate time interval because the use of such exhaust gas purifiers for a long period of time leads to a reduced purifying performance. Regeneration of DPF is accomplished by increasing DPF temperature to combust and remove the collected PM. The increase of DPF temperature is achieved by injecting fuel into the exhaust gas and oxidizing the injected fuel by an oxidation catalyst located upstream of the DPF for increasing exhaust gas temperature. Regeneration of DPNR system is accomplished by injecting fuel into the exhaust gas to reduce the stored NOx in a rich air-fuel ratio.
There is also known an exhaust gas recirculation device or EGR device by which part of the exhaust gas discharged from the combustion chamber of the engine is recirculated to flow from the exhaust passage into the intake passage in order to reduce the amount of NOx emission from the internal combustion engine. The EGR device has an EGR passage and an EGR valve provided in the EGR passage for adjusting the flow rate of EGR gas. The opening of the EGR valve is controlled so that the EGR rate of the intake gas flowing into the combustion chamber has an optimal value as determined in accordance with the engine operating condition. As an example of such EGR device, there is a device that recirculates EGR gas from the exhaust passage downstream of the exhaust gas purifier into the intake passage.
In the internal combustion engine having such EGR device and the exhaust gas purifier, when fuel is injected into the exhaust gas for regeneration of the exhaust gas purifier during the recirculaion of EGR gas into the intake passage, carbon dioxide concentration and oxygen concentration in the exhaust gas are changed because of the fuel combustion in the exhaust gas purifier, and correspondingly carbon dioxide concentration and oxygen concentration in the recirculated EGR gas are also changed. This causes a difference in the EGR rate between the current value and the target value after the fuel is supplied for regeneration of the exhaust gas purifier, thereby resulting in an unstable combustion in the engine.
To prevent such problem, Japanese Unexamined Patent Application Publication No. 2008-208723 discloses an internal combustion engine in which air flow rate that is correlated to the oxygen concentration is monitored in the intake passage where the EGR gas is introduced, and the opening of the EGR valve is controlled so that no change occurs in the air flow rate after the regeneration of the exhaust gas purifier. Such structure prevents combustion in the engine during the recirculaion of EGR gas from becoming unstable due to the change of the oxygen concentration in the EGR gas caused by catalytic combustion of the fuel injected into the exhaust gas for regeneration of the exhaust gas purifier.
The amount of unburned component in the exhaust gas, such as THC and CO, discharged from the combustion chamber of the internal combustion engine is changed depending on the engine operating condition. For example, when the accelerator pedal is depressed quickly for acceleration of the vehicle, the amount of air and fuel supplied into the combustion chamber is increased for increasing the output of the engine. Although the amount of fuel injected from the injector into the combustion chamber can be changed quickly in response to the change in the target command value, the amount of air introduced into the combustion chamber does not necessarily reach the target value quickly because there is a time lag until the boost pressure reaches the target value and also until the amount of air reaches the target value in response to the change of the throttle valve opening. Such time lag may cause increase of unburned component in the exhaust gas discharged from the combustion chamber.
The unburned component in the exhaust gas is oxidized in the exhaust gas purifier provided in the exhaust passage, and correspondingly the carbon dioxide concentration and the oxygen concentration in the exhaust gas are changed. In the internal combustion engine disclosed in the publication No. 2008-208723, the air flow rate in the intake passage is calculated after and before the fuel is injected into the exhaust gas for regeneration, and the EGR valve opening is adjusted based on the calculated values. However, such EGR control is not based on the change of the oxygen concentration and the carbon dioxide concentration when no regeneration is performed. Thus, when the oxygen concentration and the carbon dioxide concentration in the exhaust gas are changed by the increase of the unburned component in the exhaust gas caused by the factors such as the change in the engine operating condition other than the regeneration of the exhaust gas purifier, the combustion in the engine becomes unstable.
The present invention is directed to providing an internal combustion engine that maintains stable combustion even when EGR gas composition changes due to fuel combustion in an exhaust gas purifier of the engine.